This study is in patients with neuropathic pain taking gabapentin or pregabalin, and will test the clinical relevance of these preclinical data by comparing placebo to the cholinesterase inhibitor. Investigators focus not only on mechanistic hypotheses in the laboratory studies, but also on practical applications, using clinically approved drugs, including gabapentin and pregabalin to activate noradrenergic activity and donepezil (Aricept®), approved for the treatment of Alzheimer's dementia, but not previously tested to treat neuropathic pain, to inhibit cholinesterase. Each of these drugs may act by mechanisms in addition to those involved in descending noradrenergic inhibition, but investigators hypothesize that the therapeutic strength of their combination relies heavily on this cascade engendered by noradrenergic sprouting and altered α2-adrenoceptor function. The proposed studies will provide critical tests of this hypothesis and critical information to guide more effective clinical therapy of neuropathic pain.